Excited and ionized states of aniline: Symmetry adapted cluster configuration interaction theoretical study

Singlet excited states and ionized states of aniline are studied by the symmetry adapted cluster/configuration interaction method. Absorption bands of states that have mainly $π–π*$ nature are assigned as $A″1$ $(∼1B2),$ $A′1$ $(∼1A1),$ $A″1$ $(∼1B2),$ $A′1$ $(∼1A1),$ $A″1$ $(∼1B2)$ in increasing-energy order. An $s$-Rydberg state is predicted to lie between the first and second valence states, in agreement with recent experimental results. The lowest band has a charge-resonance character with a slight charge-transfer (CT) character (CT is defined as $NH2→C6H5);$ third and fifth valence bands have back-CT (BCT) nature, and second and fourth are local excitations within the benzene ring. The extent of CT of excited states depends on amino group conformation. In the planar form, CT characters of several states were altered; however, spectral shapes are very similar to that of the equilibrium form. On the other hand, amino group twisting altered both the spectrum and nature of excited states. Third and fourth lowest valence states exhibited strong CT character, while fifth to eighth states are of the strong BCT type, implying that the CT nature of excited states of aniline can be changed by amino group twisting. For ionized states, the lowest three states are assigned to $A′2$ $(∼2B1),$ $A″2$ $(∼2A2),$ $A′2$ $(∼2B1)$ in increasing-energy order, all being π-ionizations. The sixth one is also due to π-ionization $(∼2B1)$ and the others are σ-ionizations. Ordering was the same as Koopmans’ case.